Sheet detection apparatus

ABSTRACT

Apparatus which inactivates a xerographic machine in response to a copy sheet misspuff. A sheet of paper remaining on the xerographic drum beyond the stripping station causes a change in light intensity at a photocell which receives reflected light from the drum. This intensity change results in a change in resistance within the circuit to cause a current flow through a capacitor. This, in turn, energizes a threshold switching means to energize a control relay to stop the machine.

United States Patent [72] inventors Robert A. Domalski Rochester; Richard C. Kahler, Onatrio; Donald J. Quant, Rochester, all of, NY. [21] Appl. No. 841,516 [22] Filed July 14, 1969 [45] Patented July 13, 1971 [73] Assignee Xerox Corporation Rochester, N.Y.

[54] SHEET DETECTION APPARATUS 2 Claims, 3 Drawing Figs. [52] U.S.Cl 317/124, 250/206, 250/219,307/117 [51] lnt. Cl G01n 21/30 [50] Field of Search 250/219 DF, 219, 206; 317/124; 307/1 17 [56] References Cited UNITED STATES PATENTS 2,859,960 11/1958 Magondeaux 317/124 X 3,360,652 12/1967 Bemous 3,393,604 7/1968 Lundin Primary ExaminerRobert K. Schaefer Assistant ExaminerWilliam J. Smith Anomeys-Paul M. Enlow, James .1. Ralabate, Norman E.

Schrader, Ronald Zibelli and Michael J. Colitz, Jr.

317/148.5X 3l7/124X PATENTEDJULISIHH 3,593,065

SHEET 1 BF 2 By DONALD ATTORNEY SHEET 2 UF 2 PATENIED JUL 1 3 1971 L L 2m mm mm 8 L Em 2m m I -16 3% Nam mmom mo w 95 90 =5 mum 21 ME mwm mmm w 9m wmm mm omm NEW 8 1 mum SHEET DETECTION APPARATUS This invention relates to apparatus for inactivating a machine in response to an improper condition and in particular to a circuit including a photoconductive cell which detects the presence of a sheet of paper on a rotating xerographic surface and inactivates the apparatus upon the sheet being at an undesired location.

In a transfer type xerographic machine, a rotating drum having a photoconductive surface is used to create an electrostatic latent image on its surface. A powder image in the configuration of the electrostatic latent image is developed on the drum surface and transferred to cut sheets of paper or other support material by means of an electrostatic charge applied to the paper. Since there is a latent electrostatic charge on the surface of the drum and an electrostatic charge is applied on the paper during the transfer operation, the paper tends to adhere to the surface of the xerographic drum. The sheets of paper are stripped from the surface of the drum after transfer, usually by a puffer which blows a sharp steam of air between the paper and the drum and physically forces the sheet of paper away from the drum surface. Occasionally, a sheet of paper will adhere to the drum surface past the position at which it should be removed. This occurs when, for any reason, the puffer mechanism fails to operate or the paper is not aligned in the proper position on the drum to be removed from the drum. When this situation occurs, and the misspuff is not detected by other sheet sensing elements, it is desirable to detect the sheet of paper on the drum surface and immediately shut down the machine before the sheet is moved into the developer housing to cause loss of developer therefrom.

The present invention permits the utilization of a light source directed against the surface of the xerographic drum with the light reflected into a photocell to detect the presence or absence of paper on the drum surface and to shut down the machine upon the detection of a sheet of paper.

According to this invention, a light source is directed against the xerographic plate so that a portion of the light is received by the photocell. When the machine is operating normally, the light source reflects light from the drum to the photocell to retain a control relay deenergized and the machine running. When a sheet of paper adheres to the surface of the drum beneath the photocell, the light is reflected from the paper to rapidly increase the light reaching the photocell. The associated circuitry is designed such that when the amount of light reaching the photocell varies significantly, the photoconductive cell responds to rapidly change the characteristics of the circuitry and shuts down the machine.

Prior art devices, as described for example in US. Pat. No. 3,360,652 to T. Bernous, achieve this end with circuitry which inactivates the xerographic machine when reflected light reaching a photocell increases or decreases beyond a predetermined absolute level. Consequently, a change in light intensity received by the photocell may undesirably inactivate the machine as when residual toner images pass beneath the photocell, when the photocell becomes dirty or when the intensity of the lamp van'es. The present apparatus is insensitive to such changes since the machine is inactivated through a rapid change of light received by the photocell. This rapid change causes a current flow through a capacitor to activate a control relay and inactivate the machine.

It is, therefore, an object of this invention to control a machine in response to changes in light intensity reaching a photocell.

It is a further object of this invention to detect sheets of paper which adhere to a rotating xerographic drum.

It is a further object of this invention to detect misfed sheets of paper in a xerographic apparatus.v

These and other objects of this invention are attained by means of a photocell responsive to changes in light reflected from misfed sheets of paper to energize a control relay which effects the operation of a xerographic machine.

For a better understanding of the invention as well as other objects and further features thereof, reference is had to the following detailed description of the invention to be read in connection with the accompanying drawings, wherein:

H6. 1 is an isometric view of part of a xerographic machine including a xerographic drum with a light source and a photocell mounted adjacent thereto;

HO. 2 is an isometric view of the light source and photocell shown in FIG. 1; and

FIG. 3 is an electrical schematic diagram of the circuit including the photocell sown in FIG. 1.

As shown in the drawings a xerographic member in the shape of a drum 10 is shown in the position that it would be mounted in an automatic xerographic machine. The drum is rotatably mounted on a shaft, not shown, extending through bearing 12 in the middle of the drum. The drum is held on the shaft by means of a hand nut 14, which cooperates with threads on the end of the shaft to secure the drum in place. Sheets of paper are fed by the conveyor 16 into surface contact with the drum and electrostatically tacked to the drum surface by transfer corotron 18. At this point, xerographic powder images which have previously been developed on the drum surface are transferred to the surface of the sheet of paper by the transfer corotron 18. As the drum continues to rotate, the sheet of paper is carried by the drum towards the conveyor belt 20. The conveyor 20 is a vacuum conveyor which will hold the paper on the surface of the belt and draws the paper away from the drum surface after the sheet has contacted the surface of the conveyor.

As the leading edge of the sheet of paper rotates to a position just over the conveyor belt 20, a puffer tube 22 directs a series of sharp streams of air through nozzle 24 between the drum surface and the sheet of paper. The noules 24 extend across the length of the drum and effectively deflect the sheet of paper from the surface of the drum down against the surface of the belt 20. The stream of air in the puffer tube 22 emerging from the nozzles 24, is produced by a small air pump, not shown, and controlled and timed by electrical circuitry, not shown, to puff at the proper time to deflect the sheet of paper into the conveyor belt. Further details of the puffer and other portions of the xerographic machine may be had by reference to US. Pat. No. 3,301,126 issued to Osborne et al.

If, for any reason, the puffer mechanism fails to strip the sheet of paper from the surface of the drum, the sheet of paper would continue to rotate around the drum surface interfering with the operation of other mechanisms about the periphery of the drum and causing leakage of developer from its housing. Also, with the sheet of paper on the surface of the drum, further xerographic images cannot be produced or developed. It is, therefore, desirable to immediately stop the machine and manually remove the sheet of paper from the drum surface. For this purpose a photocell and light source support 26 is mounted adjacent to the xerographic drum 10 immediately after the puffer tube 22 and before a precleaned corotron 25.

The support 26 is a bracket which retains an electroluminescent panel 28 and photocell 30 in proper orientation with respect to each other and with respect to the xerographic drum. The panel has a transparent window 32 in the center thereof to permit light, reflected from the drum, to be received by the photocell, which is located behind the window portion of the panel. The support 26 is formed with a lip 34 for holding the electroluminescent panel in place, a central plate section 36 for holding the photocell in place and an apertured support section 38 to permit its mounting in the machine. Electrical leads 40 and 42 couple the photocell to its associated circuit and the panel to a source of potential.

The electrical circuit of which the photocell 30 is a part and which may be considered element PC] of the circuit, is shown in FIG. 3. The circuit also includes a main DC power source PS1 for supplying the powerto the circuit and a control relay Kl which, when deenergized permits the xerographic reproducing machine to run,'and which, when energized, inactivates the machine.

The circuit may be considered as being made up of four major sections, the misspuff section, the clear section, the startup timer section and the dark history section. The first section, the misspuff section, includes the control relay Kl, a threshold switching means in the form of silicone controlled rectifier SCR1, transistors Q2 and Q4, capacitor C4 and the photocell PCl. The purpose of this section is to pennit the normal operation of the machine when the control relay K1 is in a first or deenergized state but to switch the control relay to a second or energized state to stop the machine upon the sensing of a misspuff by the photocell.

When the photocell detects a misfed sheet, the resistance across the photocell decreases sharply. This drop in photocell resistance will cause an increase in voltage between pin 6 and 4 due to the divider network including R1 and PC]. C2, placed across PCl, bypasses transients from PCI to ground. This increase in voltage will permit current flow through C4 which provides enough power to the base of O2 to turn on 02 through R19, R3 and C9. C9 is to prevent any transient from triggering 02. Once Q2 turns on it will then provide the bias current for Q4 through R7 and R8. C8 is to provide additional transient immunity to the circuit. As soon as Q4 turns on it will provide current through R9 and R10 which will provide the gate voltage to turn on SCRl. Once SCRl turns on, we complete the path through Q5, R26, SCRl and diodes CR6 and 7 to turn on and hold in the relay Kl which will shut the machine down in the misspufi" jam. R11 provides additional current to maintain a sufficient voltage drop across SCRl. CR12 prevents the field from collapsing across the relay and assists in pulling it in.

Small changes of light intensity caused by dust, residual toner images on the drum, etc., may cause a change in photocell resistance and a slight current flow through C4. Minor flows of current through C4 caused by such conditions, however, are insufficient to turn on Q2 and inactivate the machine.

Now tat we have the machine locked up in the misspuff jam the only way to reset the jam is by removing the paper. Removing the paper will cause a decrease of light at the photocell and a corresponding large increase in resistance across the photocell. This large increase in resistance will cause a decrease in voltage between 6 and 4. This decrease in voltage will permit current flow through C3 which will provide current through R2 and R18 to the base of Q] to turn it on. Once Q1 turns on, it will provide current through R4 and R to turn on 03. C7 is again provided to add additional transient immunity to the circuit. Once 03 turns on the voltage across SCRl is clamped down such that we back bias the SCRl or commutate it and turn it off. The relay K1 will remain energized by current flow through 2K2 and Q3 only until 03 has turned off. Once K1 is deenergized the circuit is reset so that the machine can again function normally.

In the normal operating state as the machine begins to print, voltage is applied across pins 1 and 4 through the DC power source PS1. As soon as the start print button is depressed on the machine, power is applied across pins 1 and 4. Current flows through diode CR which will prevent a negative voltage from entering the circuit and false triggering it. Positive transients are shunted through C1] to prevent false triggering. Once we have voltage applied to the circuit, we have two timers which start their timing simultaneously. The first timer is the major delay timer which prevents any initial power surge transients from false triggering the circuit. This will activate this portion of the circuit after a predetermined time delay, as for example one second, by charging up C6 through R20 and R17 to the firing potential of the unijunction transistor ()6. R15 is to provide temperature compensation for Q6. Once Q6 fires, this will provide a signal through R14 and across R16 to the gate of SCRZ and once SCR2 fires it will provide current through R12 and R13 to the base ofQS which will turn on Q5. This will thus now allow the circuit to detect paper on the drum. CR1] is to provide the system with a one shot capability such that the unijunction transistor, once it has fired, will not free run and fire again. This prevents any false triggering on a rapid startup.

The second timer is the dark history timer. The presence of this section is needed due to the fact that when the photocell is dark for any prolonged period of time, as for example several hours, it has a memory to the dark and requires a long length of time to stabilize its resistance. This time is in excess of the time allowed by the startoff timer section thus necessitating the need for reducing this stabilizing time for the photocell. When a potential is placed across the photocell the input capacitors C3 and C4 are charged up to a high voltage very quickly by the circuit section. After this predetermined time delay period has ended, the charging network is switched out and the photocell is switched in. More particularly, the starting up of the machine causes C10 to charge up to the PS1 potential through R23 24, 25. This will turn off Q7 once the voltage across CR9 has been reversed and will thus remove the voltage divider network R22, R21 and CR8 from the system which had previously applied sufficient voltage to the input capacitor C3 and C4. This causes C3 and C4 to charge up quickly to a voltage determined by divider R21 and R22. After 07 fires and drops out the voltage divider network, the current through C3 and C4 is small enough that it cannot trigger the firing circuit unless a misspuff occurs.

Contact 2K2is provided to eliminate the problem of a jam shut down caused by a toner banding on the drum. Toner banding is a strip of toner located axially on the drum surface and occurs after a jam. This would normally come around under the photocell after the jam was cleared at which time the photocell would view the dark band which would cause a sudden increase in resistance such that it would allow current to flow through C3 which would turn on the Q1 and Q3. This would cause the relay to trigger through 05, relay K1, R26 03. This would provide a parallel holding path for the relay. Under this condition, the relay would pull in but then drop out. The relay, however, would have been held in for a sufficient time to cause the machine to shut down. If it were to shut down in this manner, there would be no paper on the drum and it would cause a false shutdown. Relay 2K2, however, is in the circuit to prevent such false shutdowns. This contact is normally closed but is opened during machine printing times so that the control relay cannot be energized by current flowing through this path. In this manner, the problem of toner banding is overcome.

While the instant invention as to its objects and advantages is shown in a specific embodiment hereof, it is not intended to be limited thereby, but it is intended to be protected broadly within the scope of the appended claims.

What we claim is:

1. Apparatus for use in detecting misfed sheets in an automatic xerographic reproducing machine and for inactivating the machine in response to detected misfed sheets including,

control means adapted to pennit the o era on of the machine when said control means is in a first state and to shut down the machine when said control means is in a second state,

threshold switching means coupled with said control means to control current flow through said control means,

a light source positioned in the xerographic reproducing machine, photocell means electrically coupled with said threshold switching means and positioned in the xerographic reproducing machine to receive light from said light source and capacitor means coupled to said threshold switching means and said photocell means to prohibit the flow of current to said threshold switching means during normal operatons of the reproducing machine so that the control means may be held in its first state, the capacitor means also being adapted, when the resistance of said photocell means is rapidly decreased by the sensing of a misfed sheet in the xerographic reproducing machine to cause a flow of current to said threshold switching means whereby said threshold switching means conducts current to render said control means in its second state to thereby inactivate the xerographic reproducing machine.

2. A circuit for use in detecting mist'ed sheets in an automatic machine including adapted, when said photocell means changes resistance through the detection of a misfed sheet, to cause a circuit imbalance and flow current through said capacitor whereby said threshold switching means is energized to reverse the state of said control means,

a voltage divided network to provide a stabilizing voltage to said capacitor for a predetermined time upon energization of the circuit,

transistor means in series with said control means and said threshold switching means, said transistor means being in a nonconducting state prior to activation of the circuit and means to energize said transistor means a predetermined time after the energization of the circuit to thereafter permit current flow through said control means. 

1. Apparatus for use in detecting misfed sheets in an automatic xerographic reproducing machine and for inactivating the machine in response to detected misfed sheets including, control means adapted to permit the operation of the machine when said control means is in a first state and to shut down the machine when said control means is in a second state, threshold switching means coupled with said control means to control current flow through said control means, a light source positioned in the xerographic reproducing machine, photocell means electrically coupled with said threshold switching means and positioned in the xerographic reproducing machine to receive light from said light source and capacitor means coupled to said threshold switching means and said photocell means to prohibit the flow of current to said threshold switching means during normal operatons of the reproducing machine so that the control means may be held in its first state, the capacitor means also being adapted, when the resistance of said photocell means is rapidly decreased by the sensing of a misfed sheet in the xerographic reproducing machine to cause a flow of current to said threshold switching means whereby said threshold switching means conducts current to render said control means in its second state to thereby inactivate the xerographic reproducing machine.
 2. A circuit for use in detecting misfed sheets in an automatic machine including control means adapted to permit operation of the machine when in a first state and to shut down the machine when in a second state, a threshold switching means coupled with said control means to control current flow through said control means, photocell means electrically coupled to said threshold switching means, capacitor means coupled to said threshold switching means and said photocell means to prohibit the flow of current to said threshold switching means during normal operations of the machine, said capacitor means also being adapted, when said photocell means changes resistance through the detection of a misfed sheet, to cause a circuit imbalance and flow current through said capacitor whereby said threshold switching means is energized to reverse the state of said control means, a voltage divided network to provide a stabilizing voltage to said capacitor for a predetermined time upon energization of the circuit, transistor means in series with said control means and said threshold switching means, said transistor means being in a nonconducting state prior to activation of the circuit and means to energize said transistor means a predetermined time after the energization of the circuit to thereafter permit current flow through said control means. 